(1) Field of the Invention
This invention relates to a diversity apparatus for performing a synchronous detection of a received signal by reproducing a carrier wave.
(2) Description of the Prior Art
Conventionally, digital communication apparatuses transmit information signals by modulating carrier-wave signals based on digital information signals (baseband signals) to achieve efficient transmission. There are such modulation methods as: Amplitude Shift Keying (ASK) for modulating the amplitude of the carrier-wave signal based on the digital baseband signal (modulating signal), Frequency Shift Keying (FSK) for modulating the frequency of the carrier-wave signal based on the modulating signal, Phase Shift Keying (PSK) for modulating the phase of the carrier-wave signal based on the modulating signal, and Quadrature Amplitude Moduration (QAM) for modulating the amplitude and phase of the carrier-wave signal based on the modulating signal.
There are such known methods for demodulating digital signals modulated by the above methods as the synchronous detection, the delay detection, and the frequency detection. Among these methods, the synchronous detection may gain the best receiving performance since the method reproduces the carrier wave from the received signal to demodulate the modulated signal.
In the synchronous detection, the method of reproducing the carrier wave is the most important factor in making demodulated signal since the performance depends on the ability of reproducing the carrier wave.
There are such methods of reproducing carrier waves as: the Costas method in which the frequency of the received signal is multiplied; the reverse modulation method in which demodulated results are returned as feedback; and the re-modulation method. The reverse modulation method and re-modulation method more have been put into practical use in non-fading line such as in satelite communications since these methods are able to reproduce the carrier wave even with low reception level compared to the Costas method.
It is well known in digital modulation methods that the reception performance, when applied to mobile communications and the like, deteriorates seriously due to the fading, a phenomenon in which reception level (RSSI: Received Signal Strength Indicator) changes seriously when affected by reflected or scattered electric waves branch.
The diversity reception, in which a plurality of receiving lines are used for reception, has been put into practice as an effective method to compensate for the reception level deterioration caused by the fading. It is therefore possible to improve the performance under fading by combining the diversity reception with the synchronous detection.
It is considered that conventional diversity apparatuses reproduce carrier waves for each receiving line in order to perform the diversity after the synchronous detection.
However, with such a construction diversity is performed after synchronous detection is performed for each receiving line, and the carrier wave reproduction cannot make the most of the improvement achieved with the diversity. That is, though the conventional construction of diversity apparatus has improved the reception sensitivity compared to an apparatus using only the synchronous detection, it still shows deteriorated reception performance when deterioration in Carrier/Noise (C/N) ratio of the received signal causes the deterioration in the carrier wave reproduction ability. Therefore, the carrier wave reproduction ability should be improved, especially for a case where a great number of receiving lines are combined. The more the number of combinations in diversity is, the apparatus is operated under worse C/N ratio.
The above problem has made it difficult to apply the synchronous detection to mobile communications and the like since the synchronous detection requires a stable reproduction of the carrier wave.